1. Field of the Invention
This invention relates to a transparent conductive oxide (TCO) layer for a light admitting electrode in a photoelectric device, and more particularly to a method for forming the TCO layer with a rough surface texture.
2. Description of the Prior Art
A layer of TCO such as indium tin oxide (ITO) or tin oxide (SnO.sub.x) is used as a light admitting electrode in a photoelectric device. Particularly in an photoelectric conversion device, it is desired that the transparent front or light facing electrode admit as much light as possible to the semiconductor layer in which light energy is converted into electric energy.
A photovoltaic device which comprises a TCO front electrode, a semiconductor layer, and a back electrode stacked on a transparent insulative substrate such as a glass plate is described, for example, in U.S. Pat. Nos. 4,064,521 and 4,281,208. Generally, the TCO front electrode is formed by a vacuum evaporation method, an electron beam evaporation method, a sputtering method, a CVD method, a spray method or the like, and comprises a single layer or stacked layers of TCOs such as ITO and SnO.sub.x.
Referring to FIG. 1, there is shown a schematic sectional view of a TCO layer thus formed. Deposited on a glass plate 1a is a TCO layer 2a comprising aggregated small TCO particles having a mean particle size in the range of about 200-2000 .ANG.. Accordingly, the surface of the deposited front electrode shows only a small unevenness of about 200-1000 .ANG. in height with a periodicity of about 500-2000 .ANG. and thus is substantially flat.
The refractive index of the TCO layer is about 2.0, while the refractive index of a semiconductor layer which is to be formed on the TCO layer is generally much larger than 2.0. For example, the index is about 4.0 for amorphous silicon derivatives such as amorphous silicon, amorphous silicon carbide, and amorphous silicon germanium. Accordingly, incoming light is partly reflected at the interface between the front electrode and semiconductor layer, and thus the quantity of light entering the semiconductor layer for photoelectric conversion is decreased.
Considering that the reflection characteristic at the interface between the front electrode and semiconductor layer is strongly influenced by the state of the interface, attempts to increase the quantity of light entering the semiconductor layer have been made by increasing the unevenness of the interface.
Firstly, an attempt to form a front electrode having a rougher surface has been made by increasing the TCO particle size. With a mean TCO particle size of about 2000-10000 .ANG., for example, a rough surface with an unevenness of about 1000-5000 .ANG. in height and a periodicity of about 2000-10000 .ANG. can be obtained. However, since such a TCO layer with a large mean particle size results in an increase in the electric resistivity, in a decrease of the light transmissibility, and in a decrease of the adhesiveness to the substrate, it is not suitable for the front electrode.
Referring to FIG. 2, there is shown a schematic sectional view of a front electrode having an uneven surface, which was recently disclosed by Hyodo et al. in the Japanese Patent Laying-Open Gazette No. 175465/1985 and in "Extended Abstracts" for the 32nd Spring Meeting of the Japan Society of Applied Physics and Related Societies, 1985, 29p-U-14. An unevenness texture 1tex is formed first on a surface of a substrate 1b, and a front electrode 2b is then formed with the usual small TCO particles on the uneven surface. Accordingly, this TCO layer 2b does not cause any increase of the resistivity, nor any decrease of the transmissibility, nor any decrease of the adhesiveness, and thus it is usable as a front electrode. However, it is difficult to process the substrate surface so that it will have an unevenness of a few thousand angstroms in height, and the process is not suitable for mass production.